Dehydrochlorination catalyst

ABSTRACT

Increased selectivity to vinylidene chloride is obtained in the catalytic vapor phase dehydrochlorination of 1,1,2-trichloroethane by conducting the reaction in the presence of a cesium nitrate catalyst.

BACKGROUND OF THE INVENTION

It is known to crack or thermally dehydrochlorinate1,1,2-trichloroethane to obtain 1,1-dichloroethylene (vinylidenechloride) and an isomeric mixture of cis- andtrans-1,2-dichloroethylenes. Vinylidene chloride may be eitherpolymerized to produce polyvinylidene chloride resin or may becatalytically hydrochlorinated to produce methylchloroform, a commonlyused degreasing solvent. Since the 1,2-dichloroethylene isomers are ofconsiderably less value, the vapor phase reaction is typically conductedin the presence of catalysts which selectively favor the formation ofvinylidene chloride.

U.S. Pat. No. 3,870,762 discloses dehydrochlorinating1,1,2-trichloroethane in the presence of a chloride or fluoride ofpotassium, cesium or rubidium carried on a silica support of a typeknown as Porasil®. Copending, commonly assigned application Ser. No.744,564, filed Nov. 24, 1976, discloses a vinylidene chloride selectivecatalyst comprising cesium halide, preferably cesium chloride, supportedon humidified, high surface area silica gel.

A catalyst has been found which gives improved selectivity to vinylidenechloride in the vapor phase dehydrochlorination of1,1,2-trichloroethane, especially when the vapor phase reaction isconducted at super-atmospheric pressure.

SUMMARY OF THE INVENTION

1,1,2-Trichloroethane is dehydrochlorinated in the vapor phase in thepresence of a catalyst comprising cesium nitrate and more particularlycesium nitrate supported on humidified, regular density silica gel.

DESCRIPTION OF THE INVENTION

Generally, the catalyst of the invention is prepared by contactinghumidified, regular density silica gel with a cesium nitrate solutionand drying the cesium nitrate impregnated silica gel.

The term "silica gel" generically includes a wide variety of materialshaving a wide range of physical properties and compositional limits. Asused herein, "silica gel" refers specifically to amorphous, regulardensity silica gel having a high surface area. More particularly, thesilica gel for use in accordance with the invention has a BET surfacearea of at least 500 and typically between 600 and 800 square meters pergram, an average pore diameter of between 10 and 100 angstroms and asilica (SiO₂) content of at least 99.5 percent of weight on an anhydrousbasis. The silica gel usually contains trace amounts of metal oxides,such as aluminum, iron and sodium oxides, in amounts usually not inexcess of about 0.2 percent by weight on an anhydrous basis. The boundwater content of the silica gel (which is substantially equivalent tothe loss on ignition at 950° C.) is in the range of 5 to 14 percent byweight.

Silica gel, when contacted with liquid water or lower alkyl alcoholsreadily disintegrates into small particles often with the rapidevolution of heat. Due to this disintegration, it is difficult toprepare a catalyst support of a predictable size by contacting thesilica gel with an aqueous solution of the catalytically activematerial. To prevent disintegration when contacted with an aqueoussolution of the catalytically active material (in this instance, cesiumnitrate), the silica gel is first subjected to a humidificationtreatment.

Humidification is effected by exposing the silica gel to water vapor,preferably by contacting the silica gel with a gas such as air ornitrogen saturated with water vapor. The humidification temperature isnot particularly critical, however, the temperature should not be so lowas to result in ice formation or so high as to tend to desorb moisturefrom the silica gel. Preferably, humidification is effected at atemperature of between 5° C. and 50° C.

The adsorption by the silica gel of water vapor from the saturatedcarrier gas is gradual with no apparent exothermicity such as isobserved when the silica gel is contacted with liquid water. Dependingon conditions, the silica gel may be sufficiently humidified in aslittle as one hour or as long as one month. Although to effecthumidification it is sufficient to simply expose the silica gel to watervapor laden carrier gas at atmospheric pressure, humidification may beeffected more rapidly by passing the vapor laden carrier gas underpressure through a bed of silica gel.

The degree of humidification will vary depending on the precisecomposition and physical properties of the silica gel, however, a simpletest determines when the silica gel is sufficiently humidified, i.e., asample of the silica gel is contacted with liquid water at ambienttemperature, and if there is no significant disintegration, the silicagel is adequately humidified.

After the silica gel has been humidified, it is contacted with anaqueous solution of cesium nitrate at such concentration and for a timesufficient to impregnate the silica gel with the desired quantity ofcesium nitrate. The cesium nitrate impregnated silica gel is then driedto remove excess moisture and is ready for use. Although the cesiumnitrate content of the catalyst is not particularly critical, generallyuseful levels for vapor phase dehydrochlorination are in the range of0.5 to 25 percent and preferably between 5 to 15 percent and mostpreferably about 10 percent by weight cesium nitrate based on thecombined weight of cesium nitrate and silica gel.

In a typical practice of the invention, 1,1,2-trichloroethane isdehydrochlorinated in the vapor phase by contacting a vaporous feed of1,1,2-trichloroethane with a catalyst comprising cesium nitratesupported on humidified, regular density silica gel. Thedehydrochlorination is conducted at elevated temperature, at atmosphericor super-atmospheric pressure and for a time sufficient to effect thedesired degree of conversion of 1,1,2-trichloroethane.Dehydrochlorination in accordance with this invention is preferablyconducted in accordance with the process described in copending,commonly assigned application Ser. No. 852,899 filed Nov. 18, 1977.Generally, according to the process described in application Ser. No.852,899 a vaporous mixture of 1,1,2-trichloroethane and a nonreactivediluent is fed to a reactor at a pressure of from 2 to 20 atmospheres,preferably from 4 to 10 atmospheres, the partial pressure of1,1,2-trichloroethane in the vaporous mixture being preferablymaintained at less than about 2.0 and most preferably at about 1.0atmosphere although the 1,1,2-trichloroethane partial pressure could bemaintained at sub-atmospheric levels. The vapor phase reaction istypically conducted at a temperature of at least about 375° C. andusually between 400° C. and 500° C., the contact time of the vaporousmixture with the catalyst in the reactor being not more than about twominutes and preferably not more than about 10 seconds. The catalyst maybe used as a packed or fluidized bed. Although particle size is notparticularly critical, for use as a packed bed, the catalyst particlesize is typically in the range of 6 to 50 mesh (U.S. Sieve) and in therange of about 50 to 200 mesh (U.S. Sieve) for use as a fluidized bed.

The invention is further illustrated but is not intended to be limitedby the following examples.

EXAMPLE 1

A quantity of silica gel (W. R. Grace & Co., Grade 40) having a particlesize of 6 to 12 mesh (U.S. Sieve) and a surface area of about 700 squaremeters per gram was placed in a porcelain evaporating dish exposed tothe atmosphere. A stream of ambient temperature (about 23° C.)compressed air saturated with water vapor by means of a bubbler assemblywas directed into the mass of silica gel. The silica gel wasperiodically tested for adequacy of humidification by placing a samplein water at ambient temperature to determine the extent ofdisintegration. After four weeks contact with the water vapor laden air,the silica gel was determined to be adequately humidified according tothe water contact test, i.e., the silica gel particles did notdisintegrate when placed in liquid water.

About 142 grams of the humidified silica gel was placed in a one literround bottom flask. About 16 grams of cesium nitrate (CsNO₃) wasdissolved in 200 milliliters of warm water and added to the flask. Theflask, containing the silica gel and cesium nitrate solution was rotatedin a water bath maintained at about 55° C. for several hours until thecesium nitrate solution was absorbed by the silica gel. The contents ofthe flask were removed, placed in an oven, and dried overnight at 110°C., cooled to room temperature and stored for use. The catalystcontained about 10 percent by weight of cesium nitrate based on thecombined weight of cesium nitrate and silica gel.

EXAMPLE 2

The procedure of Example 1 was followed except that 142 grams ofhumidified silica gel was treated with a solution of 16 grams of cesiumcarbonate (Cs₂ CO₃) dissolved in 200 milliliters of water. The catalystcontained about 10 percent by weight of cesium carbonate based on thecombined weight of cesium carbonate and silica gel.

EXAMPLE 3

The procedure of Example 1 was followed except that 142 grams ofhumidified silica gel was treated with a solution of 16 grams of cesiumchloride (CsCl) dissolved in 200 milliliters of water. The catalystcontained about 10 percent by weight of cesium chloride based on thecombined weight of cesium chloride and silica gel.

EXAMPLE 4

The procedure of Example 1 was followed except that 142 grams ofhumidified silica gel was treated with a solution containing 16.7 gramsof cesium chloride (CsCl) and 8.4 grams of potassium fluoride (KF)dissolved in 200 milliliters of water. The catalyst contained about 10percent by weight of cesium chloride and about 5.0 percent by weight ofpotassium fluoride based on the combined weight of cesium chloride,potassium fluoride and silica gel.

EXAMPLE 5

A vertical reactor was constructed from a 70 cm. length of 2.54 cm. I.D.Inconel® schedule 40 pipe. A preheater comprising a 30.5 cm. length of2.54 cm. I.D. schedule 40 nickel pipe was joined at a right angle to andadjacent the upper end of the reactor pipe. The reactor was charged withthe catalysts prepared in Examples 1 to 4 to a depth of about 25.4 cm.,the catalyst bed supported on a porous silicon carbide disc mountedabout 15.2 cm. from the lower end of the reactor pipe.

Heat was supplied to the reactor via a 30.5 cm. tube furnace disposedabout that section of the reactor pipe containing the catalyst. Thepreheater was heated by a resistance heater made by covering thepreheater pipe with a layer of glass cloth and wrapping there around alength of No. 22 nichrome wire.

The reactor and preheater were stabilized at operating temperature andliquid 1,1,2-trichloroethane was metered into the preheater wherein itwas vaporized and the vaporized 1,1,2-trichloroethane was fed undersuper-atmospheric pressure downwardly through the particular catalystbed and the emergent gas stream was collected and analyzed. Two runswere made with the cesium nitrate catalyst and one run each with theother catalysts. The results and conditions of these dehydrochlorinationexperiments are summarized in Table I.

As shown by the data in Table I, vapor phase dehydrochlorination of1,1,2-trichloroethane in the presence of cesium nitrate results ingreater catalytic selectivity to vinylidene chloride with higher productratios of vinylidene chloride to 1,2-dichloroethylene isomers, ascompared with other cesium-containing catalysts.

Although the invention has been described with reference to cesiumnitrate and in particular to cesium nitrate supported on humidified,regular density silica gel, cesium nitrate may be unsupported orsupported on other materials, e.g., silica, silica gels other than theregular density type, alumina, clay minerals, and the like. Moreover,other metal nitrates, e.g., potassium nitrate, sodium nitrate, rubidiumnitrate and lithium nitrate are also believed to be suitable for use inthe practice of this invention in its broadest aspects.

                  TABLE I                                                         ______________________________________                                        Vapor Phase Dehydrochlorination of 1,1,2-Trichloroethane                      at Elevated Pressure With Cesium-Containing Catalysts                         Run No.      1       2       3      4    5                                    ______________________________________                                        Reactor Temperature,                                                                       415     446     437    450  444                                   ° C.                                                                  Reactor Pressure,                                                                          46      45      53     45   41                                    psig.                                                                        Contact Time,                                                                               9      10      11      7   11                                    seconds                                                                      Catalyst     CsNO.sub.3                                                                            CsNO.sub.3                                                                            Cs.sub.2 CO.sub.3                                                                    CsCl CsCl                                                                           KF                                  TCE conversion,                                                                            90      95      82     78   41                                    percent                                                                      Selectivity to VDC,                                                                        39.8    41.0    37.9   34.4 36.1                                  percent                                                                      Selectivity to c-1,2,                                                                      31.2    31.3    33.0   31.5 35.0                                  percent                                                                      Selectivity to t-1,2,                                                                      26.5    25.7    26.6   24.8 24.5                                  percent                                                                      Yield VDC/Pass,                                                                            35.8    39.0    31.1   26.8 14.8                                  percent                                                                      Ratio VDC/c-,t-1,2                                                                         0.69    0.72    0.64   0.61 0.61                                 ______________________________________                                         TCE = 1,1,2-trichloroethane?                                                  VDC = vinylidene chloride                                                     c-1,2 = cis-1,2-dichloroethylene                                              t-1,2 = trans-1,2-dichloroethylene                                            CsNO.sub.3 = cesium nitrate                                                   Cs.sub.2 CO.sub.3 = cesium carbonate                                          CsCl = cesium chloride                                                        KF = potassium fluoride                                                  

I claim:
 1. A catalyst for use in catalyzing vapor phasedehydrochlorination of 1,1,2-trichloroethane to produce vinylidenechloride comprising cesium nitrate deposited on particulate supportmaterial, said catalyst having a cesium nitrate content of from 0.5 to25 percent by weight based on the combined weight of cesium nitrate andsupport material wherein the support material is silica gel having a BETsurface area of at least 500 square meters per gram, an average porediameter of between 10 and 100 angstroms, a particle size in the rangeof from 6 to 200 mesh, and a silica content of at least 99.5 percent byweight on an anhydrous basis, said silica gel having been humidified,prior to depositing cesium nitrate thereon, by contacting the silica gelwith water vapor at a temperature between 5° C. and 50° C.
 2. Thecatalyst of claim 1 wherein said cesium nitrate composition of thecatalyst is from about 5.0 to 15 percent by weight based on the combinedweight of cesium nitrate and silica gel.